June 14, 2024


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Tim Southee, Jimmy Neesham and Henry Nicholls will all line up for the Black Caps. PHOTO: GETTY IMAGES

Black Caps are familiar to many as a summer visitor who sings from scrubby cover or barks out its alarm call when people stray too close to a nest with well-developed chicks. Others, however, are not surprised to see it at garden feeding stations during the winter months, where it looks oddly out of place among finches and titmice.

Over the past 60 years, Blackcaps from central Europe have been wintering in urban areas of Britain with increasing frequency instead of migrating south-west to Spain as they did in the past. A new migration strategy has been shown to be genetically encoded. It is maintained on breeding grounds through reproductive isolation (birds wintering in Britain mate with other birds that winter in Britain) and fitness (birds fledge more chicks during the winter in Britain).

What is the reason for the increase in Blackcaps wintering in Britain?

The winter distribution maps from BTO-led Atlas projects show that our wintering population has increased and that our winter range has expanded. However, the core distribution of the species is centred in the south and west of Britain, where winter weather conditions are more favourable. This is supported by data from the BTO Garden BirdWatch program. Climate change has resulted in milder winters in Britain over time, which could have contributed to the increase in overwinter survival rates of Blackcaps.

It is possible that the introduction of commercial wild bird foods has also contributed to the evolution of the new migration strategy since observations of Blackcaps wintering in British gardens coincided with the introduction of commercial bird foods. Researchers have unravelled the underlying mechanisms driving this change in migration routes by analysing data collected by the BTO’s Garden BirdWatch program. It was found that both supplementary food and temperature influence Blackcap occupancy rates in winter gardens. There was a greater frequency of blackcap sightings at sites that provided food more frequently and showed a preference for sites with a hotter local climate. Interestingly, supplementary feeding has become more strongly associated with Blackcap occurrence over time.

Evidence of BTO

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In this study, the underlying mechanisms that have influenced migratory behaviour among Blackcaps are examined for the first time directly. The reliability of that provisioning is influencing Blackcaps’ distribution at a national level as it has become increasingly associated with the provision of supplementary foods in British gardens over the past 12 years. Blackcaps’ expansion into Britain may have also been enabled by an improving winter climate.

As blackcaps have migrated to Britain in winter, their phenotypes and genetic makeup have diverged from those of Spanish- wintered Blackcaps, consistent with recent evidence that the species is adapting its feeding habits to exploit human-supplied food. The beaks of blackcaps wintering in Britain are narrower and longer than those of birds wintering in Spain, indicating a more general diet for migrants from the United Kingdom. Observations made by Garden BirdWatch show that bird preferences include fats and sunflower hearts.

How will the future unfold?

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Some migrants could develop sedentary behaviour due to climate change, with an associated improvement in wintering conditions further north – think of the increasing numbers of Chiffchaffs wintering in Britain rather than migrating south.  This raises the question oIf any of our breeding Blackcaps that winter around the western end of the Mediterranean start wintering here instead of heading south, this raises the question of whether they will start wintering here.y been six recorded instances of BIn the past six years, there have only been six records winter. There is also the possibility that the central Europe Depending on local weather and the availability of supplementary food, it is also possible that central European Blackcaps visiting us may increasingly stay on their breeding grounds in central Europe.gardens is rather different from what we see herIn Britain, the pattern of food provision in gardens is quite different from what we see in Germany.

According to the BTO study, human activities can play a significant role in shaping the evolutionary trajectory of wild bird populations, and the Blackcap’s response to these opportunities may be particularly important in the long run. Changes in environmental conditions could threaten the viability of traditional wintering areas in southern Spain, making new opportunities further north that much more appealing.